A modulation and demodulation technology based on overlapped multiplexing (OvXDM: Overlapped X Division Multiplexing) includes a plurality of specific implementation solutions, for example, a modulation and demodulation technology based on overlapped time division multiplexing (OvTDM: Overlapped Time Division Multiplexing), a modulation and demodulation technology based on overlapped frequency division multiplexing (OvFDM: Overlapped Frequency Division Multiplexing), a modulation and demodulation technology based on overlapped code division multiplexing (OvCDM: Overlapped Code Division Multiplexing), a modulation and demodulation technology based on overlapped space division multiplexing (OvSDM: Overlapped Space Division Multiplexing), and a modulation and demodulation technology based on overlapped hybrid division multiplexing (OvHDM: Overlapped Hybrid Division Multiplexing).
It should be noted that in OvXDM mentioned in this application, X represents any domain, for example, time T, space S, frequency F, code C, and hybrid H.
The following provides brief description by using OvTDM as an example.
First, time division (hereinafter referred to as TD) multiplexing (TDM: Time Division Multiplexing) is a technology in which a plurality of signal symbols occupying relatively narrow time durations share one relatively wide time duration in digital communication. FIG. 1 is a schematic diagram of a conventional time division multiplexing technology.
In FIG. 1, time durations (referred to as timeslot widths in engineering) of multiplexed signal symbols are respectively T1, T2, T3, T4, . . . , and in engineering, the signal symbols usually occupy a same timeslot bandwidth. ΔT is a minimum guard timeslot, and an actual guard timeslot width should be larger. ΔT should be greater than a sum of a transition time width of a used demultiplexing gate circuit and a maximum time jitter of a system. This is a most common time division multiplexing technology. This technology is used in most existing systems such as multichannel digital broadcast systems and multichannel digital communications systems.
A most significant feature of this technology when it is applied to digital communications is: Multiplexed signal symbols are fully isolated from each other in terms of time, without mutual interference. The multiplexed signal symbols are not limited, and symbol durations (timeslot widths) of signals may have different widths. In addition, this technology is applicable to different communications mechanisms, provided that timeslots of the multiplexed signal symbols do not overlap or cross with each other. Therefore, this technology is most widely used. However, such multiplexing has no effect in improving spectral efficiency of a system.
Therefore, a conventional idea is that adjacent channels do not overlap in time domain, to avoid interference between the adjacent channels. However, this technology limits improvement of spectral efficiency. An idea of a time division multiplexing technology in the prior art is that channels do not need to be isolated from each other and may strongly overlap with each other. As shown in FIG. 2, in the prior art, overlapping between channels is considered as a new coding constraint relationship, and corresponding modulation and demodulation technologies are proposed based on the constraint relationship. Therefore, a technology is referred to as overlapped time division multiplexing (OvTDM: Overlapped Time Division Multiplexing). In this technology, spectral efficiency increases proportionally with a quantity K of times of overlapping.
Referring to FIG. 3, an overlapped time division multiplexing system includes a transmitter A01 and a receiver A02.
The transmitter A01 includes an overlapped time division multiplexing-based modulation device 101 and a transmission device 102. The overlapped time division multiplexing-based modulation device 101 is configured to generate a complex modulated envelope waveform carrying an input signal sequence. The transmission device 102 is configured to transmit the complex modulated envelope waveform to the receiver A02.
The receiver A02 includes a receiving device 201 and a sequence detection device 202. The receiving device 201 is configured to receive the complex modulated envelope waveform transmitted by the transmission device 102. The sequence detection device 202 is configured to perform data sequence detection on the received complex modulated envelope waveform in time domain, to perform decision output.
Usually, the receiver A02 further includes a preprocessing device 203 between the receiving device 201 and the sequence detection device 202, configured to assist in forming a synchronously received digital signal sequence in each frame.
In the transmitter A01, the input digital signal sequence forms, by using the overlapped time division multiplexing-based modulation device 101, transmit signals that have a plurality of symbols overlapped in time domain; and then the transmission device 102 transmits the transmit signals to the receiver A02. The receiving device 201 of the receiver A02 receives the signals transmitted by the transmission device 102. The signals form, by using the preprocessing device 203, digital signals suitable for the sequence detection device 202 to detect and receive. The sequence detection device 202 performs data sequence detection on the received signals in time domain, to output a decision.
Referring to FIG. 4, the overlapped time division multiplexing-based modulation device 101 (OvTDM modulation device) includes a waveform generation module 301, a shift module 302, a multiplication module 303, and a superimposition module 304.
The waveform generation module 301 is configured to generate, based on a design parameter, an initial envelope waveform whose waveform is smooth in time domain.
The shift module 302 is configured to shift the initial envelope waveform in time domain at a preset shift interval based on a quantity of times of overlapped multiplexing, to obtain shifted envelope waveforms at fixed intervals.
The modulation module 305 is configured to convert an input digital signal sequence into a signal symbol sequence represented by using positive and negative symbols.
The multiplication module 303 is configured to multiply the signal symbol sequence by offset shifted envelope waveforms at fixed intervals, to obtain modulated envelope waveforms.
The superimposition module 304 is configured to superimpose the modulated envelope waveforms in time domain, to obtain a complex modulated envelope waveform carrying the input signal sequence.
FIG. 5 is a block diagram of the preprocessing device 203 of the receiver A02.
The preprocessing device 203 includes a synchronizer 501, a channel estimator 502, and a digital processor 503. The synchronizer 501 implements symbol time synchronization of received signals in the receiver. Next, the channel estimator 502 estimates a channel parameter. The digital processor 503 performs digital processing on received signals in each frame, to form a digital signal sequence suitable for the sequence detection device to perform sequence detection and receive.
FIG. 6 is a block diagram of the sequence detection device 202 of the receiver A02.
The sequence detection device 202 includes an analysis unit memory 601, a comparator 602, and a plurality of retained path memories 603 and Euclidean distance memories 604 or weighted Euclidean distance memories (not shown in the figure). In a detection process, the analysis unit memory 601 makes a complex convolutional coding model and a trellis diagram of the overlapped time division multiplexing system, and lists and stores all states of the overlapped time division multiplexing system; the comparator 602 finds, based on the trellis diagram in the analysis unit memory 601, a path with a minimum Euclidean distance or a weighted minimum Euclidean distance to a received digital signal; and the retained path memories 603 and the Euclidean distance memories 604 or the weighted Euclidean distance memories are respectively configured to store a retained path and an Euclidean distance or a weighted Euclidean distance that are output by the comparator 602. One retained path memory 603 and one Euclidean distance memory 604 or weighted Euclidean distance memory need to be prepared for each stable state. Preferably, a length of the retained path memory 603 may be 4K-5K. Preferably, the Euclidean distance memory 604 or the weighted Euclidean distance memory stores only a relative distance.
In an OvXDM system, a signal transmitter modulates a signal and then sends a modulated signal to a signal receiver, and the signal receiver demodulates the modulated signal after receiving it. A demodulation process includes a decoding step (that is, the sequence detection step performed by the foregoing sequence detection device). In conventional decoding, a node in a folded tree diagram (Trellis diagram) needs to be continuously accessed. In addition, two memories are disposed for each node. One is configured to store an Euclidean distance of a relative best path for reaching the node, and the other is configured to store the relative best path for reaching the node. For a system in which a quantity of times of overlapping is K and whose modulation dimensionality is M, a quantity of nodes in the trellis diagram is MK. Each node needs to be extended in a decoding process. Therefore, the quantity of nodes determines decoding complexity, and the decoding complexity increases exponentially with the quantity of times of overlapping. It is well-known that, in the OvXDM system, spectral efficiency increases as the quantity K of times of overlapping increases, and therefore the quantity K of times of overlapping needs to be increased as far as possible. However, in a conventional decoding algorithm such as Viterbi decoding, when the quantity of times of overlapping increases to a specific value (K>8), the decoding complexity increases sharply. An existing decoding method cannot meet a real-time decoding requirement, and the spectral efficiency and a decoding rate become contradictory. Therefore, the decoding complexity needs to be reduced, and decoding efficiency needs to be improved.